The present invention relates to a fusion implant, in particular for the replacement of a vertebra or an intervertebral disk, which comprises a biocompatible material having reinforcing fibers.
U.S. patent application 2001/0014826 A1 describes a space holder for a vertebra or an intervertebral disc. It comprises a jacket made of titanium and having a first and second edge to engage adjacent bone structures to be fused. Titanium implants like this can resist the load and provide initial stability. They meet the short term requirements, but not all mid and long term requirements. In particular they do not assure maximum bone fusion from one healthy bone structure to the other and they inhibit certain radiological assessments and therapies.
U.S. Pat. No. 5,306,310 discloses a prothesis as a vertebral replacement element that is tubular and made of helical strands of carbon fiber reinforced composite material. The fibers are running in the longitudinal direction of the strands.
U.S. Pat. No. 5,192,327 discloses implants stackable together to allow variability of ultimate implant height. The implants are made of a radiolucent carbon fiber reinforced polymer.
EP 1 236 451 discloses a medical implant made of fiber reinforced plastic, wherein the fibers are oriented in relationship of the biochemical requirements to obtain the appropriate strength and stiffness.
WO 02/076316 discloses a modular implant for fusing adjacent bone structure. It is made of ring-like segments and has an internal cavity for the reception of bone growth inducing substances.
There are, therefore, fusion implants available, which can be easily assembled, inserted and secured to replace a vertebra or an intervertebral disc and which bear the load normally carried by vertebral structure, which have been removed. These implants mostly fulfill the short term requirements.
The purpose of a spinal fusion is for two vertebra to grow together and form one healthy bridge of bone, which regenerates itself through the natural physiological process over time. This is achieved by placing bone generating substances between two healthy vertebra and holding them in place with anchors, such as screws, rods and plates. Interbody devices exist to hold the bone generating substances, but also to serve s a mechanical strut during the healing and bridging process.
Inert stability of the fixation and therefore the short term requirement is the major goal, if the patient cannot expect to live very long. But patients today live longer and expect to remain active. Tumor or fracture patients can expect to live for decades and return to a normal life. Fusion implants must therefore last up to sixty years. Yet, any inert device, whether it is metal, plastic or composite will wear out over time and if overstressed, fracture. On the other hand, a living and healthy bone bridge will continue to rebuild itself for the rest of the patient's life. A fusion implant should therefore not only provide initial support for corrected bone, but also enhance the generation of a stable bone bridge between the adjacent bone structures to be fused. That will track natural vertebrae functions as closely as possible.
It is an object of the present invention to provide a fusion implant that not only meets the short term requirements of stability, the mid term requirements of bone healing but also the long term mechanical requirements to assure the structural regeneration of bone. According to the invention a fusion implant made of composite material having reinforcing fibers is provided, wherein the fibers are long fibers and wherein the majority of these long fibers have a direction that follows the longitudinal forces upon said first and second faces, which is the direction of trabecular structures of the adjacent bone structures, so that said majority of long fibers are an extension of these trabecular structure. The implant according to the present invention is able to mimic the complex structure of a bone structure and in particular a vertebra and therefore imitate the mechanical properties for site-specific bone regeneration. The majority of the long fibers are oriented in a longitudinal axis, which is head to toe according to the site specific stress, in the same direction as the trabecular alignment of the vertebral body. These fibers are in fact an extension of these trabecula.
The invention considers the following properties of bone:                a) Bone is anisotropic        b) Bone regenerates itself through the patient's life according to the stress upon it        c) Bone without stress resorbs and looses its density and ability to resist stress and bear load or disappears all together.        
An inert mechanical implant wears through fatigue over time. Living bone, like other tissues, when properly stressed, regenerates and strengthen over time.
The fusion implant according to the present invention provides not only the specific mechanical resistance to allow stability, but through orientation of the fibers better achieves the site specific anisotropic properties of a bone required to maintain it healthy. The implant replicates the micro movement or contortion and the site specific, anisotropic mechanical qualities of the vertebrae. It is stiffer and stronger in one direction than another, according to the specific requirements of load bearing bone the implant must replace.
The trabeculars of the bone are regenerated and oriented according to the stress received. As the trabeculars demonstrate the complex multi-directional forces the bone receives, these can be used to orient the long fibers.
The implant as described herein not only replicates the outer anatomical form of the bone structure to be replaced, but also its inner mechanical structure to imitate the performance of the surrounding tissue. The implants are not only strong enough to resist collapse, but are also flexible enough to better propagate forces through the surrounding bone, which promotes bone healing and regeneration.
Composite material containing long fibers is uniquely suited to work with bone for the generation of new bone and remodeling. A remolding occurs, when the vascular support remains intact and the bone begins to bear load. The new bone tissues recognize the forces of the load and lay down trabecula. Trabecula are beams or planks in the support structure of a bone and are oriented according to the load they receive. Trabecula receiving load are reinforced and those that do not are weakened or removed. With the implant according to the present invention the new bone growth can be stressed as it transfers stress with the proper orientation of long fibers. The implant is flexible enough so that it will not unduly isolate stresses from the bone growth material as is the case with the known stable but stiff implants. It is just as strong as known implants but is able to transmit stress to the bone within the component because of its limited and controlled flexibility.
The invention permits the manufacture of an assembly of two or more biocompatible structures that allows a lordotic curve. Such an assembly allows to curve the orientation of the fibers and to mate with the orientation of the predominant weight bearing trabeculars.
Other advantages and features of the present invention will be apparent to those skilled in the art after reading the following specification with references to the accompanying drawings.